(1) Field of the Invention
The present invention relates to an exhaust gas purifying system and an exhaust gas purifying method both suitable for use particularly in a diesel engine.
(2) Description of Related Art
Heretofore, there has been known a technique wherein an oxidation catalyst and a particulate filter (hereinafter referred to simply as “filter”) are disposed in an exhaust passage of a diesel engine, allowing a particulate matter (PM) contained in exhaust gas to be deposited on the filter, and the PM thus deposited on the filter is burnt to regenerate the filter continuously.
In the exhaust gas purifying system constructed as above, NO contained in exhaust gas is oxidized in the oxidation catalyst to produce NO2, then the NO2 and the PM on the filter are reacted with each other, allowing the PM to burn (be oxidized) to regenerate the filter continuously. NO2 exhibits a high function as an oxidizing agent in comparison with NO and permits the PM to be oxidized with a relatively low activation energy (that is, permits the PM to burn at a relatively low temperature).
In a certain operating condition of an engine there is a case where the exhaust gas temperature does not rise to an activation temperature of the oxidation catalyst, so that NO is not oxidized and hence a continuous regeneration of the filter is not effected. In such a case it is necessary to perform a forced regeneration different from the continuous regeneration.
As a method for the forced regeneration there is known a method wherein a heat source such as an electric heater is attached to the filter and an electric current is supplied to the heater, allowing PM to burn, or a method wherein fuel (HC) is fed to the oxidation catalyst and is allowed to undergo an oxidation reaction in the oxidation catalyst, the temperature of the filter is increased by the reaction heat to cause burning of the PM (see, for example, Japanese Patent Laid-Open No. H07(1995)-259533).
More particularly, according to the technique described in the above literature, if the temperature of a filter (a filter with catalyst) is not higher than an activation temperature (e.g., 250° C.) of the catalyst, an additional fuel (first additional fuel) is injected in an amount proportional to the filter temperature. The injection timing of this additional fuel is relatively earlier than a final stage of the expansion stroke. By injecting fuel at such a timing, the additional fuel and a high-temperature combustion gas present within the cylinder are mixed together, so that the additional fuel burns in the intake port and in the exhaust passage and the exhaust gas temperature rises.
As a result of the high-temperature exhaust gas being fed to the catalyst, the catalyst temperature rises, and when the catalyst temperature rises to its activation temperature, an additional fuel (second additional fuel) is further injected also in the expansion stroke in addition to the first additional fuel described above. The second additional fuel injected in the expansion stroke reaches the catalyst without burning in the cylinder and in the exhaust port, and is burned in the catalyst whose temperature has reached the activation temperature. As a result, the filter located downstream of the catalyst is heated up to a temperature which permits oxidation of PM, whereby the combustion of PM (regeneration of the filter) is executed.
According to the technique disclosed in the above literature, the amount of the first additional fuel to be injected is changed according to the filter temperature, but with such a method, it is difficult to hold the filter temperature stably in the temperature region (600° C. and thereabouts) in which the PM burns most efficiently.
That is, according to the technique disclosed in the above patent literature, the amount of the first additional fuel to be injected is changed according to the filter temperature for the purpose of maintaining the catalyst temperature at its activation temperature, but once the catalyst temperature reaches the activation temperature, the quantity of the second additional fuel is set to a predetermined quantity. It is the second additional fuel quantity that exerts a great influence on the filter temperature, but according to the technique disclosed in the above patent literature it is impossible to control the filter temperature because the second additional fuel quantity is set constant. If the filter temperature is too high, PM burns in a short time and the filter temperature becomes a still higher temperature, with consequent likelihood of fusion of the filter, while a too low temperature will result in a defective regeneration.
It may be effective to control the additional fuel injection quantity in accordance with the engine speed and engine load, but also by this method it is impossible to control the filter temperature to an optimal temperature.